Let me start off by saying that when I first got into manga and this was ongoing, it was absolutely one of my favorite series. It's very cute and lighthearted and there are a lot of classic shoujo tropes (the poor hard working tsundere protagonist, the rich popular loner love interest, etc.). I enjoy the Usui/Misa relationship a lot because I'm all about that oblivious/pining/possessive love - and they do it really well 🙂 . The art is nice too, and the maid thing provides ample excuse to do different costumes and theme days where all the outfits are just super cute and fun.
As others have said though, if you're looking for a good story with complexity and layers and character growth, this is not the manga for you. Not only do Misaki and Usui not evolve as individuals, their relationship doesn't really evolve either - the feelings don't change or deepen, Misaki just gains the ability to actually be open about said feelings (...sort of). Even the many side characters, despite showing up a lot, don't seem to grow much. And while the plot in the beginning is mainly episodic and straightforward, the more serious storyline that starts about halfway through the manga feels contrived, confusing, and in general poorly executed. It's really too bad, because there were definitely real opportunities for depth that the manga did not take advantage of.
I'm thinking specifically of Aoi-chan's crossdressing not being accepted by his father, and how at some point in the future he apparently...just gives up doing it? Also Misaki's father coming back could have been much more important and hurtful and a cause for growth - but instead his back story is sort of rewritten and he becomes a comedic character instead.
Rating-wise, part one (pre-chapter 33) gets a solid 7 or maybe even 8 for cuteness and nostalgia, while the latter half gets a 4 (5 if i want to be nice and give credit for the manga at least trying to do something different). So ultimately, I'm giving this a 6.
tl;dr: A good choice if you're new to manga and/or looking for something easy to read. Consider just watching the anime instead, which covers the good parts and stops before the bad.